1. Field of the Invention
The present invention relates to surveying probes and methods and, more particularly, it pertains to a surveying probe and method which are suitable for computing positions of difficult-to-reach points.
2. Description of the Prior Art
Conventional surveying devices generally include a vertical pole having a prism mounted at a top end thereof and a bottom tip adapted to be placed on a target point whose position is to be determined. A total station including a theodolite and an electronic measuring device targets the prism to determine the spatial coordinates thereof. By holding the pole exactly vertically over the target point and by knowing the distance separating the prism from the tip of the pole, the spatial coordinates of the target point may be readily computed.
Accordingly, to determine the position of a target point which is difficult to reach from above, for instance, a point located at the bottom of a building wall, it is required to first determine the coordinates of at least two surrounding vertically accessible points. Afterwards, direction measurement of the vectors extending between the surrounding points and the target point is used to locate the target point. This method of ascertaining the position of a non-vertically accessible point is time consuming and not very efficient in that the vertical surveying pole and the associated total station must be installed at two different locations to obtain the coordinates of the two surrounding points. Moreover, it has been found that this method does not always provide accurate results.
U.S. Pat. No. 5,512,905 issued Apr. 30, 1996 to Nichols et al. discloses a vertical surveying pole which is adapted to compensate for relatively small out-of-plumb conditions of the pole during measurement taking. The vertical surveying pole is equipped with a GPS unit adapted to determine the position of a GPS antenna disposed at a predetermined distance from a target point, a tilt sensor for providing information about the degree of angular tilt of the pole, and a direction sensor for providing information about the direction of any such tilt. On the basis of the position of the GPS antenna and the information provided by the tilt and direction sensors, the position of the target point may be more accurately computed.
U.S. Pat. No. 4,509,269 issued Apr. 9, 1985 to Leitz discloses a target device for marking points on industrial objects, such as an airplane fuselage, when the sight line and the object surface to be measured form an acute angle. The Leitz's device comprises a rod having a longitudinal axis passing through a target point contained in the object surface. The rod is provided at a lower end thereof with external threads for mounting the rod at a threaded bore whose position is to be determined. Accordingly, in use the rod is normal to the object surface containing the point to be measured. The rod is provided with a pair of spaced-apart mark balls which are disposed on the longitudinal axis of the rod at known spacing from each other and from the target point whose coordinates are to be determined. The coordinates of the mark balls are determined by two spaced angle measuring instruments. Knowing the distance between the mark balls and the distance separating at least one of the mark balls from the target point, the spatial coordinates of the latter may be readily calculated.
Although the devices described in the above-mentioned Patents perform satisfactorily in many applications, it has been found that there is a need for a surveying probe which can be used for computing the position of difficult-to-reach points, especially, in situations where the surveying pole has to be inclined to access the target point with the tip end thereof.